The present invention relates to the field of energy storage systems, and in particular to a method for controlling an energy storage system having battery modules coupled in parallel, in order to optimize the power utilization of the battery modules. The invention also relates to a corresponding control unit adapted to control the energy storage system.
Within the field of battery systems, energy storage systems are frequently used in a wide variety of applications and fields of technology. In, for example, the automotive industry, energy storage systems may be used for propulsion of a vehicle as well as to provide electrical energy to various systems of the vehicle.
In order to increase the power capability of the energy storage system, a solution can be provided where two or more battery modules/battery packs of the energy storage systems are coupled in parallel to each other. Hereby, the individual battery modules can be easily connected to, or disconnected from, each other. Also, an increased total capability is provided in comparison to using only a single battery module/battery pack.
However, a problem with energy storage systems having parallel coupled battery modules is that the battery modules need to be in approximately the same physical state for optimal energy usage. It is however a common scenario that the battery modules/battery packs are not in the same physical state and as an example, if the parallel coupled battery modules are differently aged, i.e. one of the battery modules has been recently exchanged with a new and unused battery module, there will most likely be a difference in power capability between the differently aged battery modules, which in turn will result in a reduced charge capability and discharge capability for the oldest battery module. If, on the other hand, one of the battery modules has a higher temperature than the other battery modules of the same energy storage system, the resistance of the battery module having higher temperature will most likely be lower than the battery module having lower temperature. Hereby, there is a risk that the warmer battery module will receive a charge current which is higher than its capability.
There is hence a need for controlling charge and discharge capability for energy storage systems having battery modules coupled in parallel.
US 2012/0119746 present a solution wherein each of the storage cells, i.e. the batteries, is connected to a respective cell controller. The respective cell controllers are in turn connected to processing and control module for measuring and storing parameters for each of the cells, such as temperature, current and voltage. Hereby, the storage cells are controlled in order to increase the overall performance of the plurality of storage cells.
Although US 2012/0119746 present a solution for the above identified problem, the energy storage system in US 2012/0119746 is still in need of further improvements in terms of e.g. cost efficiency.
It is desirable to provide a method for controlling an energy storage system having improvements in relation to prior art solutions.
According to a first aspect of the present invention there is provided a method for controlling an energy storage system, the energy storage system comprises at least two battery modules electrically coupled in parallel to each other, wherein the method comprising the steps of receiving a signal indicative of a maximum power capability of each of the respective battery modules; assigning a power threshold limit for the at least two battery modules of the energy storage system corresponding to the lowest maximum power capability received from the battery modules; and providing a charge current to the at least two battery modules, the charge current having a current level being proportional to the power threshold limit.
The wording “power threshold limit” should in the following and throughout the entire description be interpreted as a power limit which is not exceeding the power limit of the battery module having lowest maximum power capability. Also, the power capability can be measured by means of power, current or voltage. Accordingly, the present invention should not be construed as limited to measuring only one of power, current or voltage. The skilled person knows that these variables are dependent on each other and receiving a value for one of the variables when measuring one of the other variables is just a manner of simple calculation.
Furthermore, the wording “battery module” should in the following and throughout the entire description be interpreted to also include battery packs, which in themselves may comprise one or more batteries. Still further, the wording “battery module” should be understood to also include a module which may comprise a plurality of battery packs. Accordingly, the wording “battery module” may be a single battery, a battery pack which comprises more than a single battery, as well as a module which comprises more than a single battery pack.
An advantage of the present invention is that the applied charge current is directly dependent on, and proportional to, the maximum power capability of the battery module having the lowest power capability. Hereby, the battery module having the lowest power capability sets the limit for the charge current which reduces the risk of providing a charge current being damageable to the battery module. Also, by providing a charge current as described above, there is no need to individually controlling all of the at least two battery modules, i.e. it is only the battery module with the lowest power capability that is controlled while the remaining battery modules of the energy storage system receive their charge current dependent on the battery module having the lowest power capability. Hereby, a cost efficient method is provided in comparison to the above described prior art solution, since there is no need of controlling each of the battery modules.
According to an example embodiment, the method may be further followed by the steps of measuring a battery charge level of the battery module having lowest maximum power capability; and adjusting the charge current for reducing a difference between the battery charge level of the battery module having the lowest maximum power capability and the power threshold limit.
Hereby, the battery module having the lowest, power capability may be utilized to its optimum limit. As an example, if one of the battery modules in the energy storage system is exchanged with a new one, the power capability of the older battery may be smaller in comparison to the new battery module. Also, the resistance of the older battery module may, most likely, be larger than the new battery module. Hereby, when the charge current, which level is being proportional to the power capability of the older battery module, is provided to the parallel coupled battery modules, the older battery may not receive power up to its maximum power capability. In order to utilize the older battery module optimally, the charge current is adjusted, i.e. increased, such that the difference between charge current for the older battery module and the power threshold limit is reduced. Naturally, the remaining battery modules of the energy storage system, i.e. the battery modules having a larger maximum power capability than the older battery module, will also receive an increased charge current. Accordingly, the overall performance of the energy storage system will be increased.
According to another example, if one of the battery modules of the energy storage system has an increased temperature level in comparison to the remaining battery modules, i.e. it is warmer than the remaining battery modules; the resistance of the warmer battery module will most likely be lower in comparison to the remaining battery modules. When the charge current is applied there is hence a risk that the warmer battery module will receive a charge current which corresponds to a power level which is higher than its maximum power capability, i.e. the warmer battery module may receive more charge current than it can handle. This may, for example, result in the battery module being aged more rapidly. By adjusting the charge current, in this example to be lower than the originally provided charge current, will then provide for an increased operational lifetime of the warmer battery module, as well as to prevent that the temperature is increased even further. Also, the operational lifetime of the entire energy storage system may be increased.
According to an example embodiment, the step of adjusting the charge current may comprise the steps of iteratively receiving battery charge level information from the battery module having the lowest maximum power capability and adjusting the charge current until the difference between the battery charge level of the battery module having the lowest maximum power capability and the power threshold limit is within a predetermined limit value.
By iteratively receiving battery charge level of the battery module and adjusting the applied charge current based on the received battery charge level information enables the method to increase/decrease the charge current in small steps, i.e. “little by little”. Hereby, the difference between the battery charge level of the battery module having the lowest maximum power capability and the power threshold limit can be made with further accuracy.
It should be readily understood that the predetermined limit value can be set differently depending on a desired precision of the described differences between the battery charge level of the battery module having the lowest maximum power capability and the power threshold limit. Naturally, different applications are in need of different precision and will therefore not be discussed further.
According to an example embodiment, the method may be further configured to receive battery parameter status for each of the at least two battery modules of the energy storage system, such as temperature, voltage, current, state-of-charge and state-of-health.
Hereby, it is further possible to predict how much the charge current should be adjusted, i.e. increased/decreased, in order to reduce the difference between the battery charge level of the battery module having the lowest maximum power capability and the power threshold limit
According to an example embodiment, a memory unit may be connected to the energy storage system, the method may further comprise the step of providing information relating to battery parameter status and adjusted charge current to the memory unit. According to an example embodiment, the memory unit may be a PID-regulator.
Hereby, the memory unit stores the adjusted charge current as well as the various parameters of the battery modules. An advantage is that when the same, or approximately the same, scenario between the battery modules occur again it may be easier to adjust the charge current to a desired level, since the memory unit knows how much the charge current was previously adjusted. Such a scenario may, for example, be when a battery module is exchanged with a new one. The memory unit may then remember how much charge current that was applied when previously adjusting to a desired level when changing battery modules. Furthermore, a PID-regulator is well known and may hence be suitable to use.
According to a second aspect of the present invention there is provided a control unit for controlling an energy storage system, wherein the energy storage system is connected to a generator and comprises at least two battery modules electrically coupled in parallel to each other, each of the at least two battery modules is connected to a measurement unit configured to measure a battery charge level of the respective battery modules, wherein the control unit is adapted to receive a signal from each of the at least two battery modules, the signal being indicative of a maximum power capability of each of the respective battery modules; assign a power threshold limit for the at least two battery modules of the energy storage system corresponding to the lowest maximum power capability received from the battery modules; and control the generator to provide a charge current to the at least two battery modules, the charge current having a current level being proportional to the power threshold limit.
Effects and features of this second aspect are largely analogous to those described above in relation to the first aspect of the present invention.
According to an example embodiment, the control unit may be further configured to receive a measured battery charge level of the battery module having lowest maximum power capability; and adjust the charge current for reducing a difference between the battery charge level of the battery module having the lowest maximum power capability and the power threshold limit.
According to an example embodiment, the control unit may be further configured to monitor battery parameter status for each of the at least two battery modules of the energy storage system, such as temperature, voltage, current, state-of-charge and state-of-health.
According to an example embodiment, the control unit may be further configured to adjust the charge current dependent on the monitored battery parameter status.
According to an example embodiment, the control unit may further be connected to a memory unit, wherein the memory unit is connected to the energy storage system and is configured to receive and store information from each of the at least two battery modules relating to battery parameter status; and receive and store information from the control unit relating to adjustment of charge current provided to the at least two battery modules.
Further features of, and advantages with, the present invention will become apparent when studying the appended claims and the following description. The skilled person realize that different features of the present invention may be combined to create embodiments other than those described in the following, without departing from the scope of the present invention.